Not a musician but I'm agreeing with you that it seems to make sense. I know you may feel attacked for this view but I think it's a decent idea, just not sure I would bother with making the change.
There is 2 different issues here :
- You are right. Knowing the theory helps in improvising things in the real world and that could totally apply to casting with TM
- There is an balance in the game between the different type of casting and people relent on changing this equilibrium.
1 thing I can propose for you is to make it a Virtue saying that soma magician are particualry good to my Theory and improv and get a bonus from this...
I don't think @temprobe 's point is relevant to OP. That spontaneous casting is not necessary in the lab, does not mean that it is not useful. It makes a lot of sense that there are traits, maybe call them creativity, that are critical to both spontaneous casting and lab work. I reckon that's why 3ed/4ed used Int, rather than Sta, for the spontaneous casting total.
The best argument IMO is still this:
Magic Theory is, if anything, too valuable.
One could imagine breaking down the traits in even smaller and more fundamental traits, like creativity, concentration, dedication, lab procedures, experimental method, theoretical knowledge, theoretical understanding, etc. The good thing would be that we could really apply common traits in every situation where it matters. So we spontaneous magic could benefit from the theoretical understanding aspect of MT without benefiting from the lab procedures aspect. The downside would be that the game would not be playable.
I'd say: test it. Only playtest will tell if it is ok or not.
The impact, as stated by some already, is likely to be small. We are making MT, which is an already essential skill, even better? Yes. So what? Again, the impact is likely to be small.
Test it for a few sessions, then come back here and report your findings. Make sure that one of the players has LLSM and another one has Diedne magic.
I think I begin to see where you are coming from.
Music Theory should help you to compose songs and symphonies, as Magic Theory is used to compose Formulaic and Ritual spells.
By extension, skill in a particular musical instrument is similar to a score in an Art.
Your experience is that Music Theory helps produce good impromptu tunes, and I can sort of see that by analogy Magic Theory might help produce impromptu Spontaneous Magic.
But if I take that analogy to extremes, then House Bonisagus and House Verditus should produce the best sponters. Which doesn't feel quite right to me, though at this time I can't make a cogent argument against it. This requires some consideration...
One of the reasons i proposed halving the effect of MT as a portion of this new casting total addition (in other words before division) was that while theory is important in improvisation a larger part is skill with your instrument (Arts) and skill with the other musicians if there are any, which soes not have a great analog here. Though, in my mind, the second part adds weight to MTs inclusion in the total as there are not other "musicians" but factors such as the astrological influences in that particular time/place, aura, etc. one must account for. That said, even without halving it seems a minor bonus and definitely won't overshadow virtues like Diedne Magic or LLSM in most instances and, if magi with those get this benefit too, will make them even better with Sponting.
Side note, again not super relevant, but canon fluff does state that sponting in the lab is quite common and part of ones lab total but only as part of the reason why casting non-ritual, non-item effects in the lab can only help the lab total if they use different arts than those that actually make up the lab total. OTOH, the negative side of this, granting a mechanical penalty for being unable to spont in the lab, is not a thing.
Regarding the idea of MT being used to help spontaneous magic:
I have a saga rule where I allow my players to roll Int + Magic Theory to try to figure out ways to use spontaneous magic to solve a problem, though I don't allow it to add to the roll. Still, its a good excuse for the players to directly ask each other and myself for ideas how to Get Through That Wall with their Deficient Perdo.
I imagine you improvise a good deal more than I do, but I don't consciously use music theory when I do.
Do you ever use music theory when you have done? To reflect and evaluation and improve?
(From the viewpoint of learning theory, it would be a good idea to do so.)
Perhaps I should ask - do you only use Music Theory when you have 15 minutes to think about it, or when you have less than a second to come up with a tune?
The difference is not theory or not, but consciousness or not.
Thinking 15 minutes is a System 2 decision in Daniel Kahneman's terminology, that is a conscious deliberation, and everybody who knows a little theory would use it and be conscious about doing so.
Deciding in a second means a System 1 decision, which would be subconscious. It is intuitive, but it is intuition in Herbert Simon's sense which is essentially information retrieval, working off a body of experience. The process is subconscious and one does not usually know why and how one came up with the idea, and if it involved theory one might not know.
However, many skilled theoreticians use their theoretical understanding when they build up their repertoire of examples that fuel System 1. Applications of theory is part of the experience from which intuitive ideas are drawn. As is often said, one does not learn from experience, but from reflecting upon the experience, and this is where theory is a tool to improve intuition.
This gives solid theoretical and scientific basis to say that theory improves spontaneous practice, but it does of course hide the ambiguity of theoretical skill. Some have massive skill in manipulating abstract theory, others have massive skill interpreting theory in practice. The two do not always come together, but Ars Magic only has one magic theory ability.
It would certainly be sensible, but I'm lazy and music is my second love, not my first I suppose I turn to theory if I want to turn what I improvised into something more permanent; depending on how much work I do I think that's analogous to either turning a spontaneous spell into a (learned) formulaic spell or putting in the legwork in advance for a ceremonial casting.